WIP e2e key backups

Continues from Matthew's work: adds a feature flag & panel in
user settings to create a backup.

Can't restore a backup yet, nor even continue backing up to the same
backup after a refresh.

src/components/structures/MatrixChat.js

@@ -48,6 +48,8 @@ import SettingsStore, {SettingLevel} from "../../settings/SettingsStore";
 import { startAnyRegistrationFlow } from "../../Registration.js";
 import { messageForSyncError } from '../../utils/ErrorUtils';
 
+import SuggestKeyRestoreHandler from "../../SuggestKeyRestoreHandler";
+
 /** constants for MatrixChat.state.view */
 const VIEWS = {
     // a special initial state which is only used at startup, while we are

src/components/structures/UserSettings.js

@@ -736,6 +736,16 @@ module.exports = React.createClass({
                 </div>
             );
         }
+
+        let keyBackupSection;
+        if (SettingsStore.isFeatureEnabled("feature_keybackup")) {
+            const KeyBackupPanel = sdk.getComponent('views.settings.KeyBackupPanel');
+            keyBackupSection = <div className="mx_UserSettings_section">
+                <h3>{ _t("Key Backup") }</h3>
+                <KeyBackupPanel />
+            </div>;
+        }
+
         return (
             <div>
                 <h3>{ _t("Cryptography") }</h3>
@@ -751,6 +761,7 @@ module.exports = React.createClass({
                 <div className="mx_UserSettings_section">
                     { CRYPTO_SETTINGS.map( this._renderDeviceSetting ) }
                 </div>
+                {keyBackupSection}
             </div>
         );
     },